Human cyclin D1 has been associated with a wide variety of proliferative diseases but its biochemical role is unknown. In diploid fibroblasts we find that cyclin D1 is complexed with many other cellular proteins. Among them are protein kinase catalytic subunits CDK2, CDK4 (previously called PSK-J3), and CDK5 (also called PSSALRE). In addition, polypeptides of 21 kd and 36 kd are identified in association with cyclin D1. We show that the 36 kd protein is the proliferating cell nuclear antigen, PCNA. Cyclin D3 also associates with multiple protein kinases, p21 and PCNA. It is proposed that there exists a quaternary complex of D cyclin, CDK, PCNA, and p21 and that many combinatorial variations (cyclin D1, D3, CDK2, 4, and 5) may assemble in vivo. These findings link a human putative G1 cyclin that is associated with oncogenesis with a well-characterized DNA replication and repair factor.